refactor(fe): migrate plex/messaging/admin/downloads/notifications to qk (R7 S2b)

Extend lib/queryKeys.ts to the remaining ~34 key roots and migrate every
remaining literal (queryKey: [...] AND the positional useLiveQuery keys) in
36 files. All ~200 React Query keys across the app now route through the
factory; zero raw key literals remain outside queryKeys.ts.

The heterogeneous composite plex keys (plex-library/facets/collections/
playlists carry a filters object, a ratingKeys array, or a union/group
discriminator) take a variadic pass-through — the factory centralizes the
ROOT string; single-id keys stay precisely typed. Nullable keys (thread,
playlist, ytSearch) accept null so a null segment is preserved.

Pure substitution — byte-identical key arrays, so TanStack prefix matching
is unchanged. Gate green: typecheck (app+node+e2e), eslint (0 err),
prettier, 68 vitest. Runtime-verified on a fresh dev server: Feed + Plex
modules load, feed/facets/tags/my-status/plex-library/plex-facets/
plex-collections/plex-playlists all 200, full UI renders.

frontend/src/components/VideoEditor.tsx

@@ -1,4 +1,5 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import {
@@ -73,7 +74,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
   const aspect =
     (job.asset?.width && job.asset?.height ? job.asset.width / job.asset.height : 0) || 16 / 9;
   const sb = useQuery({
-    queryKey: ["storyboard", job.id],
+    queryKey: qk.storyboard(job.id),
     queryFn: () => api.downloadStoryboard(job.id),
     staleTime: Infinity,
     retry: false,
@@ -307,8 +308,8 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
       return { files: N };
     },
     onSuccess: ({ files }) => {
-      qc.invalidateQueries({ queryKey: ["downloads"] });
-      qc.invalidateQueries({ queryKey: ["download-usage"] });
+      qc.invalidateQueries({ queryKey: qk.downloads() });
+      qc.invalidateQueries({ queryKey: qk.downloadUsage() });
       notify({ level: "success", message: t("editor.created", { count: files }) });
       onClose();
     },